Thyristor converter-modulated impedance under unbalanced system conditions

The paper presents the use of a digital simulation program to assess the concept of modulated impedance under unbalanced system conditions, and the relationship between apparant system impedance Z_ac and thyristor converter rectifier firing delay angle α. Modulated impedance/reactance applications in power systems and machines include network stability enhancement tools, VAR shunt compensation, subsynchronous resonance damping, motor drive speed control, and starting of large slip-ring induction motors. The digital simulation results were validated by laboratory testing.     

Modeling and enhanced control of DFIG under unbalanced grid voltage conditions

This paper presents a mathematical model of a doubly fed induction generator (DFIG) based on stator voltage orientation (SVO) in the positive and negative synchronous reference frames under unbalanced grid voltage conditions. The oscillations of the DFIG electromagnetic torque and the stator active and reactive powers are fully described during grid voltage unbalance. A new rotor current controller implemented in the positive synchronous reference frame is proposed. The controller consists of a proportional integral (PI) regulator and a harmonic resonant (R) compensator tuned at twice the grid frequency. Thus, the positive and negative sequence components of DFIG rotor currents are directly regulated by the PI + R controller without the need of involving positive and negative sequence decomposition, which indeed improves the dynamic performance of DFIG-based wind power generation system during small steady-state and relatively larger transient network unbalances. The theoretical analysis and the feasibility of the proposed unbalanced control scheme are validated by simulation studies on a 1.5-MW wind-turbine driven DFIG system. Compared with conventional single PI current control design, the proposed control scheme results in significant elimination of either DFIG power or torque oscillation under unbalanced grid voltage conditions.     

Estimation of transmission line parameters from measurements

This paper describes the method of estimating the transmission line parameters from voltage, current, and power measurements at the two ends of the line. This becomes necessary if a new parallel transmission line is constructed between two substations due to load growth or if an underground cable replaces an overhead line and the overall parameters of the paralleled lines or the cable are desired to be determined. The parameters of the equivalent pi network of the line are obtained from the measurements at the line ends by using the Newton–Raphson method to solve the non-linear equations. The proposed method then gives the resistance/m, inductance/m, and capacitance/m, considering the distributed nature of the transmission line. The technique to determine the conductance parameter is also presented.     

STATCOM不对称控制策略下线路距离保护测量阻抗的改进算法

在电网三相电压不对称时,采用不对称控制策略能够保证静止同步补偿器（STATCOM）安全,实现良好的电压控制效果。针对较为复杂的单回并补双回线路,分析了线路短路后距离保护动作条件,证实采用不对称控制策略的STATCOM可能引起故障相保护拒动及非故障相保护误动。计及STATCOM注入电流,改进了故障测距算法。将故障距离值与线路正序阻抗配合,修正距离保护测量阻抗值。仿真结果验证了所提算法能够保证距离保护正确动作,避免接入STATCOM后重新整定距离保护。     

基于CPLD的车载电子线束系统的设计

本文介绍了基于CPLD实现车载电子线束系统的设计方法,用于解决传统汽车布线多且复杂的问题.详细论述了整个系统的工作原理,主控和终端执行部分采用RS422标准形式进行通信,并在Altera公司的EPM7256S芯片上进行硬件设计和实时运行.软件部分使用VHDL语言和模块化的编程设计方法对CPLD进行设计,使系统具备了高性能、低功耗、工作温度范围宽、体积小和低成本等特点.最后采用Quartus II环境对程序进行仿真测试,结果证明该设计方案正确可行.     

Fault location method applied to transmission lines of general configuration

Power quality is an important concern once automation is present in almost all industrial process. Since fault occurrences affects the power quality considerably, in this paper is proposed a new fault location method applied to transmission lines constituted of any configuration, as example double circuit, untransposed sections, and multiple derivations.In order to locate the fault, the method uses voltage and current phasors gathered from terminals with measures, however the method does not need these measures from all terminals.The proposed method is composed of three blocks to locate the fault, which are: Algorithm’s Main Control, Grid Scanning Process, and Objective Function’s Minimization Process.A large number of simulations were conducted and the results show the accuracy and efficiency of the method, even in cases of high impedance faults.     

基于PSCAD的特高压直流输电系统建模与仿真分析

为了对特高压直流输电系统中的故障变化特性进行准确的仿真与分析,在PSCAD/EMTDC仿真平台上,以宾金直流为例,搭建±800 k V,8 000 MW的特高压双极直流输电系统模型,主要对直流输电系统中的无功补偿设备、滤波装置、直流线路、直流控制系统进行详细建模。对于送、受端交流系统,在换流母线处对两侧实际交流系统进行戴维南等值,搭建了以直流系统为主、交流系统简化的实际系统输电模型。并对直流系统的典型故障换相失败进行仿真分析,分析了换相失败故障发生前后,特高压直流系统中主要电气量的变化。通过与实际故障录波的波形进行对比,验证了模型的可行性。着重分析了逆变侧换流母线上流入受端交流系统有功功率、无功功率的变化。研究结论为分析特高压直流换相失败故障对于实际交流系统的影响提供了技术支持。     

金属带式汽车无级变速系统建模及动态仿真分析

无级变速(CVT)控制系统的难点是实现发动机和传动系统的最佳匹配。本文针对由发动机、离合器、CVT传动系统和汽车载荷构成的汽车整体模型,建立了无级变速汽车传动系统数学模型。在此基础上建立了以最佳燃油经济性为控制目的的整车动态仿真模型,设计了参数自调整PD速比控制器,并对4种典型汽车运行工况进行了仿真和分析。仿真结果表明所建模型正确合理。     

基于PSCAD的宜华HVDC系统建模及仿真

运用成熟的电磁仿真工具PSCAD,深入研究宜都-华新高压直流输电(HVDC)工程.参考该工程的实际数据,搭建的一次侧输电系统包含了交流系统、换流变压器、滤波器等其他组件的详细模型.搭建的二次侧控制系统模型是在CIGRE测试模型控制端的基础上修改的,并且详细介绍了控制系统的逻辑和原理.基于一次和二次侧系统模型建立宜华HVDC工程的仿真模型,并对其进行正常和故障状态下的模拟实验,验证了本模型的稳定性以及本模型可准确模拟宜华高压直流输电系统,可以作为对宜华高压直流输电进一步研究的有效平台.     

特高压直流输电系统实时闭环仿真建模

仿真研究是发展高压、超高压和特高压电网的必要手段。为深入研究特高压直流输电技术和特高压交直流混联系统的特性,基于RTDS仿真器和南瑞继保研制的±800kV直流控制保护样机搭建了云广直流输电系统"孤岛"运行方式实时闭环仿真模型。为验证所建模型的有效性和可信性,与非实时电磁暂态软件PSCAD/EMTDC对单电气元件特性、云广直流输电系统稳态潮流和暂态特征进行了仿真校核和分析。校核结果显示,2种仿真工具模拟的电气元件特性、系统稳态潮流和暂态特征基本一致,从而表明了搭建的实时闭环云广直流输电系统的可用性和可信性。由于考虑了UHVDC控制与保护设备的控制作用的实时参与,所搭建的闭环云广直流仿真系统将更能反映实际交直流系统的动态特性,更具实际工程参考价值。所述的闭环云广直流输电仿真系统已成功应用于云广特高压直流输电工程相关的多项关键科研项目,从而为云广特高压直流输电工程的建设和运行提供了有力的技术支持。     

不平衡工况下链式D-STATCOM的控制策略

对工作在不平衡条件下的星型连接的链式D-STATCOM提出一种控制策略。通过等效变换,考虑未知干扰和参数摄动,建立abc坐标系下精确的数学模型。一种无源性控制与重复控制相结合鲁棒L2增益性能的新方法用于实现电流控制,重复控制用于补偿周期性干扰,无源控制用来保证重复控制的收敛性和对控制目标的跟踪,同时也补偿非周期干扰对系统的影响。直流电压控制由三层组成,第一层通过注入正序有功电流,第二层通过注入零序电压,第三层基于H桥单元之间有功能量的交换。控制策略的有效性通过仿真和在10 k V,±5 MVar的链式D-STATCOM装置工程测试所验证。     

Dissipation of the partial ground fault current across the shield wires of transmission lines

An analytical method is presented which can be used to estimate grounding effects of the shield wires, voltage transfer and distribution of ground fault currents. In the case of lines which can, as regards earthing, be treated as infinite, the method is based on relatively simple equivalent circuits obtained by applying the expression for the earthing impedance of an infinite line and summation of finite geometric series. In the case of lines which cannot be treated as infinite, use is also made of the principle of superposition and the summation of infinite geometric series. Some numerical examples are given to illustrate the practical application of the method. Finally, the method is used to derive an analytical proof of the specific property of shield wires as earthing conductors that they partially compensate for the unfavorable influence of high soil resistivity on the resistance of the earthing electrodes which they connect.     

Analysis of three-phase wound motors under unbalanced conditions

The analysis of three-phase wound motors with unbalanced supply voltages and unbalanced rotor-phase impedances is presented. The rotating field theory and coupling impedances approach are used in modelling the motor in the steady state. The effect of rotor skewing is considered. The relationships between stator and rotor quantities, in a general form, are obtained. The analysis is verified by comparison with the conventional equations for currents and torque.     

基于Ansys的高压输电塔风振特性分析

在电力大范围应用的背景下,研究输电线路的振动特性对电网整体的安全性有着极其重要的意义。本文推导了三线两塔耦合系统的力学简化模型,并以建立典型的500kV交流双回路耐张塔为对象验证了算法的正确性。利用Ansys软件建立了两塔三线的耦合模型并进行了单塔振动分析、塔—线耦合振动分析。在此基础上利用Davenport风速谱对电力塔模型节点的风载荷进行了模拟,并对耦合模型进行了动力时程分析,最终得到了节点位移曲线,验证了模型的合理有效性。最终结果表明耦合体系的振型以塔、线各自的振型为主,并出现了一部分由塔或线诱发的新振型,且同阶下耦合的振动频率要小于单塔的振动频率。最终经过理论计算与有限元分析结果的比较,证明了所推导力学模型的正确性。     

Characteristics of the magnetic field under hybrid ac/dc high voltage transmission lines

In recent years, the possibility for ac and dc transmission lines running parallel to each other, sharing the same right-of-way (ROW) or even the same-tower has increased. Design of such hybrid ac/dc networks requires a precise calculation of the magnetic field around and under them. This paper presents quantitative analysis of the magnetic field at 1-m height above ground surface for different hybrid ac/dc transmission lines. Lateral profiles for typical 275, 132 (running in Kuwait), 500, 220 and 132 kV (running in Egypt) ac transmission lines after adding bipolar dc lines are presented. The magnetic vector potential concept, as extended to multi-conductor transmission lines employing the superposition principle is used to model and calculate the RMS values of the magnetic field generated by the hybrid ac/dc lines at any point in the space. The RMS values of the field are determined directly without dividing the ac supply cycle into a sufficient number of subintervals.     

Wavelet transforms for real-time estimation of transmission line impedance under transient conditions

 Waveforms of voltages and currents in electrical power systems may include higher harmonics, transient components and random noise. For control and protection devices, it is necessary to estimate parameters of the basic waveform. The most well-known methods are based on the Fourier technique. The estimated phasors of the basic component can be applied to the calculation of the impedance of a faulty transmission line. In this paper, the current and voltage phasors were estimated using complex wavelets. The filter properties of the wavelet approach are better than those of the Fourier. Received: 20 July 2001/Accepted: 10 August 2001     

Reactive power compensation in a multi‐line system under sinusoidal unbalanced conditions

A new generalized reactive power compensation algorithm applicable to a multi‐line system under sinusoidal conditions is reviewed. This method does not require any decomposition of reactive power or non‐active current. It is shown that a realization of the compensator can be obtained by means of a simple connection of reactive elements. Accordingly, it is concluded that to compensate the reactive power is a simple circuit synthesis problem. The algorithm is illustrated by means of examples. Copyright © 2010 John Wiley & Sons, Ltd.     

Modeling and analysis of unbalanced distribution system using object-oriented methodology

An object-oriented (OO) model for three-phase unbalanced distribution system is proposed and implemented. Single-phase system components are initially modeled using object-oriented methodology. Later, it is extended to unbalanced three-phase distribution system using the “composition” technique of object-oriented design. The main features of object-oriented methodology such as “inheritance”, “aggregation”, “association” and “polymorphism” have been exploited to obtain a better software model. Three-phase distribution load flow analysis and short circuit analysis modules have been developed using the proposed object model. The performance of the developed software has been tested on IEEE 13-Node, IEEE 34-Node and IEEE 123-Node distribution systems.     

Watthour meter accuracy under controlled unbalanced harmonicvoltage and current conditions

This work presents the results of phase I of the project on the accuracy of watthour meters when subjected to closely-duplicated real-world unbalanced harmonic voltage and current conditions. Such real-world conditions involved differences in voltage and current magnitudes as well as differences in the voltage and current total harmonic distortion levels of each phase. Tests were performed on a selected sample of nine three-phase and three single-phase watthour meters. Some results showed that under a particular set of unbalanced waveforms with harmonic conditions of I_THD>80% and V_THD>2%, the percentage errors exhibited by these meters ranged from -10.09% to +0.52%     

The high-frequency phenomenon of copper clad aluminum wires which are lower the alternating current resistance than copper solid wires,and the application of copper clad aluminum wires to the electrical equipment

The paper is proposed approximate formulas of the ac resistance of lead wires and coils which made by using copper clad aluminum (CCA) wires. There are the frequency range that the ac resistance of CCA wires is lower than it of cupper solid wires for lead wires and coils of an electrical equipment. It is shown that the application of CCA wires is economical as comparing with copper solid wires, if the design is due to successful calculation in the electrical equipment. The measured resistance is compared with the formula. CCA wires is an example of multiple conductors which shall be expected to apply in future.